We have a fundamental lack in understanding the mechanism that reduces P-glycoprotein (P-gp) expression and transport activity at the blood-brain barrier in Alzheimer's disease (AD). Lack of this knowledge is a significant clinical problem since it prevents development of an effective therapy to enhance AB clearance from the brain, lower AB brain levels, and thus prevent cognitive decline in AD. The long-term goal of the investigator is to better understand the molecular mechanisms that regulate blood-brain barrier function in neurodegenerative disorders, a goal which may lead to new therapeutic strategies to treat AD. The objectives of this particular application are to identify the mechanism responsible for P-gp reduction in AD, to validate this mechanism as a target to protect P-gp, and to test a novel therapeutic strategy for restoring P-gp. Accomplishing these objectives is expected to reduce A brain levels and improve cognition in AD. Based on preliminary data, the central hypothesis is that A mediates proteasomal degradation of P-gp, that blocking proteasomal degradation protects P-gp, and that restoring P-gp levels through PXR activation reduces A brain burden and improves cognition in mice with AD. The rationale for the proposed research is that identifying the mechanism that reduces brain capillary P-gp and protecting and/or restoring P-gp to improve A brain clearance will potentially provide novel therapeutic targets to lower A brain levels in AD. To accomplish the objectives of this application, we will test our central hypothesis by pursuing the following three specific aims: 1) Identify the mechanism of A-mediated P-gp reduction at the blood-brain barrier. 2) Validate the ubiquitin-proteasome system as a target to protect P-gp in an AD mouse model. 3) Develop a therapeutic strategy to reduce cognitive decline in an AD mouse model. In Aim 1, we will inhibit the ubiquitin-proteasome system to identify the steps involved in A-mediated P-gp reduction, and determine expression, transport activity, and ubiquitination of P-gp. In Aim 2, we will treat hAPP mice with inhibitors of the ubiquitin-proteasome system, monitor P-gp expression, transport activity, and ubiquitination, and measure A brain levels. We will conduct brain perfusion to assess P-gp activity in vivo and perform tail-flick assays to determine the consequence of changes in P-gp. In Aim 3, we will con- duct a 2-year PCN-feeding study with hAPP mice to assess the long-term therapeutic effect of PXR-mediated P-gp restoration on AB brain levels. P-gp expression and transport activity, A brain load, and cognition will be periodically determined. The proposed research is innovative because it focuses on two independent strategies designed specifically to enhance A clearance from the brain in AD. The proposed research is significant because it holds the promise of two new therapeutic strategies to lower A brain burden and slow progression of AD. The proposed research is translational because drugs for either strategy, inhibition of the ubiquitin- proteasome system and PXR activation, are currently on the market, and both therapeutic strategies could potentially be translated into the clinic for the treatment of AD patients.